User positioning, or identifying the geographical location of a user equipment (UE), has been widely used by a variety of services. Some services, e.g., emergency calls, are non-commercial and required by regulatory bodies, which may also impose accuracy requirements on the service. An example of this is the Federal Communications Commission (FCC) in the US; see FCC 99-245, “Revision of the Commission's rules to ensure compatibility with enhanced 911 emergency calling systems”.
In many environments, the position of a UE can be accurately estimated by using positioning methods based on the Global Positioning System (GPS). However, GPS-based positioning may often have unsatisfactory performance in urban and/or indoor environments. A complementary positioning method could thus be provided by a wireless network. Positioning methods based on time difference of arrival measurements (TDOA) have been widely used, for example, in GSM, UMTS and cdma2000.
Positioning methods to be used in Evolved UMTS Terrestrial Radio Access Network (E-UTRAN) networks, sometimes referred to as Long-Term Evolution (LTE), have not yet been standardized but are actively discussed in 3GPP, where downlink user equipment (UE) assisted TDOA-based positioning (Observed TDOA, or OTDOA) is one of the possible candidates.
OTDOA is a downlink positioning method which exploits a so called multilateration technique to calculate the terminal position, based on TDOA measurements from three or more locations or sites. In general, multilateration, also known as hyperbolic positioning, is the process of locating an object based on time difference of arrival measurements conducted on signals between the object and three or more distinct locations. This implies that a UE needs to be able to hear, and correctly decode, signals from at least three different sites, e.g. three neighbouring base stations (eNodeBs).
In 3GPP it has been recognized that the hearability issue for signals, used for positioning measurements, need to be addressed to enable positioning services that meet the service requirements. Achieving the required accuracy is, in general, impossible with the, in E-UTRAN, currently standardized physical signals.
One problem with existing solutions is the low detection probability at low Signal-to-Noise Ratios (SNRs), which is common for signals from neighbouring cells due to low processing gain. Combined with the fact that their correlation properties are not sufficiently good to meet detection requirements, this results in inability to correctly decode the signal from a neighbour cell, in presence of a dominant interferer, e.g. close to the serving cell. Consequently, the positioning accuracy will be unsatisfactory.
In order to address the hearability issue and enhance positioning measurements, new physical signals, called positioning reference signals (PRS), in combination with low-interference subframes (LIS), during which no data is transmitted, have been proposed for the E-UTRAN.
For LTE, the PRS configuration, i.e. the transmission pattern; sequence, transmission; transmission bandwidth etc has been under discussion. It has however been agreed that cell-specific PRS patterns should allow a reuse factor of six, which is a likely solution for standardization to be specified in 3GPP TS 36.211. Thus, the PRS pattern can be decided to have a reuse factor of six. However, in real networks, a reuse factor of six may still not be enough for meeting the target positioning accuracy. Thus other interference mitigation techniques, particularly related to network planning, would be required to improve the hearability of signals from a sufficient number of distinct locations.
The strong-interferer problem typically occurs when measuring neighbour cells, which is necessary for positioning. In such cases it is not uncommon with a strong dominance of an interfering signal over the measured signal. Furthermore, the serving cell is not necessarily always the strongest interferer when measuring neighbour cells, for reasons that will be further explained below.
Another known approach to improve positioning measurements on neighbour cell is to apply idle-period downlink (IPDL) periods, which have been standardized for UMTS (3GPP TS 25.331, Radio Resource Control (RRC), Protocol Specification). With IPDL, base station (NodeB) transmissions are synchronously ceased for a short period. The idle periods are arranged in a predetermined pseudo-random fashion, according to higher layer parameters and the random pattern is known to all user equipments (UEs). The UEs are then required to perform measurements on neighbour cells during idle periods of the own base station.
There is no agreed solution to the identified problem yet for Evolved UMTS Terrestrial Radio Access Network (LTE). Adopting the UMTS IPDL solution in LTE is likely to result in unnecessary large performance degradation, which may occur due to the fact that:                data transmissions must be ceased over a larger bandwidth in LTE than for UMTS (i.e. up to 20 MHz in LTE);        control channel transmissions are affected, since during IPDL no transmissions are allowed; and also        the measurement period may become longer for cells where IPDL is applied.        
A key aspect to observe is that the serving cell is not necessarily the strongest interferer to all neighbour cells. The situation when the serving cell is not the strongest interferer may occur for the following reasons:                some of the neighbour cell measurements e.g. signal quality such as Reference Signal Received Quality (RSRQ), which may also incorporate inter-cell interference, may not necessarily lead to the selection of the best cell, when the serving cell—or serving base station—is idle or partly idle.        the UE may not be connected to the best cell all the time, e.g. due to measurement inaccuracy, which can be large. For instance, Reference Signal Received Power (RSRP) absolute inaccuracy can be ±6 dB.        UE may not be connected to the best cell, due to certain radio resource management strategies: load balancing, intra/inter-RAT admission control, non-immediate handover, heterogeneous networks with selective user subscription policy, etc.        in real networks, some cells may generate stronger interference, e.g. because of difference in the cell sizes.        
As explained above, if there is a strong interferer, other than the serving cell, hearability will be negatively impacted and the UE may, in order to perform positioning measurements, not be able to decode signals from neighbouring cells correctly. Thus, there is a need for improved positioning accuracy and/or performance enhancements.